This invention relates to the method or process of thermoforming plastic materials into a finished product with a closed chamber therein and more particularly to a new and improved method of making a plastic product having thin cross-sectioned walls that are cooperative to define a closed cavity or chamber such as a refrigerator door.
There has been a substantial growth worldwide in the plastics industry directed to the production of articles for use in the home and commerce. The techniques used in such development include blow molding, injection molding and thermoform molding. In the manufacture of such products as refrigerator doors, very little has been done to mold plastic doors because of the inherent difficulty of forming a closed chambered product. U.S. Pat. No. 4,862,577 discloses a method of manufacturing a refrigerator cabinet's outer shell by the use of heat-curable adhesives and brackets with its accompanying hardware which involves considerable hand labor. The Gould patent (U.S. Pat. No. 2,976,577) is directed to the process of making a foamed core refrigerator door in one step by the use of two opposing flat thermoplastic sheets in mold sections and adding foam forming material to the lower portion of the mold which holds the lower plastic sheets and thence on closing the mold sections utilizing the foam material through exothermic heat and pressure generated during the molding process to form the desired contoured shape to the opposed plastic sheets while simultaneously filling the central cavity with foam. This process from a practical standpoint has the limitation that the same pressure or force is applied to all surfaces which cause an immediate stretching of the material and causes a pronounced thinness at the initial areas which in applicant's case would be detrimental to a proper distribution of the plastic material. Further, such process without preheating is not feasible for most plastic materials. In applicant's process, separate stages and forces are used in the formation process which is particularly important in the molding of the inner liner of a product like a refrigerator door which has a configuration to include separate protruding abutments which requires greater control of the flow of plastic material which affects the cross-sectional thickness of the door liner. In this process applicant uses a female plug mold to initially assist in the formation of the article while distributing the plastic sheet in a manner to assure proper thickness in the necessary areas which is done in cooperation with a male mold which then applies a negative pressure to complete the molding process and finishes the distribution of the plastic material. This action is then followed by the cooperative action of a female mold and a male mold which finishes the molding of the completed door in one complete cycle thereby eliminating the many labor intensive steps that are currently being used to assemble the panels by hand. By this process, the distribution of the sheet material during the thermoforming process enhances the structural integrity of the two panels or plastic sheets since it is able to control the stretch and distribution of the flowable material in a direction to enhance the strength and rigidity of the protruding members or abutments.